Direct fixation fasteners are mainly used in transit railroad tracks to connect railroad rails to the underlying structure. These fasteners serve to three main purposes: (1) maintain integrity of track and gauge during operations; (2) reduce noise and vibration caused by train wheels; and (3) create electrical insulation between the rails and the ground. There are two main design approaches to achieve the desired properties: (1) two steel components bonded to each other by layer a rubber element in between (a.k.a. bonded direct fixation fastener) with both steel parts are also largely encapsulated in rubber during the manufacturing process; and (2) multiple elements assembled together without the incorporation the bonding process (a.k.a. non-bonded direct fixation fastener).
Bonded direct fixation fasteners (BDFFs) have been used by transit agencies in the United States for many decades. The BDFF design provides a one component solution for the unique needs of the transit railroad systems. Dynamic stiffness of the BDFFs dictates the noise and vibration mitigation performance of the BDFFs. While achieving the desired amount of noise and vibration mitigation, BDFFs must also withstand cyclic wheel loads in diverse environmental conditions for many years. Historically, BDFFs have been made using one grade of rubber and the vertical and lateral stiffness characteristics of BDFFs have been governed by a combination of the geometric details of the design and the properties of the rubber used in the BDFF.
BDFFs with low vertical stiffness are commonly sought to create softer cushions under the rails to provide higher noise and vibration reduction. When the geometrical constraints are set by the end user or track conditions, designers are usually left with softening the rubber element to achieve better noise and vibration reduction. When BDFFs are made softer in the vertical direction, they become softer in the lateral direction as well, which undesirably leads to premature failures under repeated cyclic loading and also to higher lateral deflection of the rail head.